BGP NextHop field

Unanswered Question


I was reading CCIE R&S by Wendoll Odom about BGP Nexthop, see the diagram. It first gives a problem in this scenario, "R4's route to through R2 lists R1 IP(, R4 doesn't have a route for on R1, so that route cannot be consider best by BGP."

I think R3 SHOULD HAVE a route to why not?



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Giuseppe Larosa Mon, 11/16/2009 - 14:05

Hello Han,

if IP subnet is not advertised in iBGP or in an IGP R3 doesn't know about it.

R2 would pass the eBGP route with BGP next-hop = unchanged.

possible solutions are:

advertising ip subnet in iBGP with a network command on R2;

advertising ip subnet in IGP with a network command on R2;

use of next-hop-self on R2 on session towards R3

Hope to help



thanks first.

My understanding is that:

as long as R1 advertizes to R2. R2 would pass it to all routers in its AS, right?

So, I'd think your condition wouldnt exist. "if IP subnet is not advertised in iBGP or in an IGP R3 doesn't know about it. "

the subnet should always be advertised. unless R1 intetinally not to do it by disabling "network" command.

Correct me if I am wrong.



Giuseppe Larosa Mon, 11/16/2009 - 23:26

Hello Han,

the question is arised for all other prefixes that R1 can advertise to R2 on the eBGP session.

R1 would advertise ip subnet with a BGP next-hop of, that cannot be installed in other routers in same AS of R2 because BGP next-hop is unknown.

it is a sort of dead lock: prefix should be known to accept BGP next-hop

R1 can eventually advertise as you noted but BGP next-hop attribute will be

This is not accepted as valid by R2 iBGP peers.

things are different if it is R2 to advertise

R2 is not allowed to modify the BGP next-hop over the iBGP sessions unless next-hop-self is used.

I can tell you that this is one of the first problems I had when I started to work on BGP more then 10 years ago.

This is a real problem and not a theorical issue.

Hope to help



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